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Statutory Balances and Accounting Practices (Schedule Of Statutory Net Income And Capital And Surplus) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory net income | $ 155,654 us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount | [1] | $ 235,251 us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount | [1] | $ 63,365 us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount | [1]
Statutory capital and surplus | $ 1,438,281 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance |  | $ 1,528,682 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance |  | $ 1,440,973 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance |
RBC authorized control level | 800.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets |  | 500.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets |  | 500.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets |
Minimum RBC authorized control level | 200.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets |  | 200.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets |  | 200.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets |
CAIC [Member] |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
RBC authorized control level | 386.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = mcy_CAICMember |  | 520.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = mcy_CAICMember |  |  |
[1] | Statutory net income excludes changes in the fair value of the investment portfolio as a result of the application of the fair value option under GAAP, and reflects the impacts of other differences from GAAP.

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- Definition The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.23(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 24 -Subparagraph c -Article 7 + Details
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- Definition Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.23(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 24 -Subparagraph c -Article 7 + Details
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- Definition The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958570-112826 + Details
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- Definition Tier 1 capital divided by risk weighted assets as defined by regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958568-112826 + Details
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